HP-25 Co-luminous segments

[Tom Flatterhand]

My HP-25 recently purchased on an auction platform appears to be in good shape. However, it exhibits a phenomenon: when I press the keys on the middle row of the keyboard, all of the upper segments, except for the fourth, of the 7-segment digits light up dimly in the display. My meager electronics knowledge suggests that one of the keyboard's matrix wires (if it works that way) has a weak short to another wire. Do any of you have a concrete idea what this could be and if this quirk can be fixed?

   

[teenix]

The LEDs for digit number 4 are energized when key column 4 is selected from the Cathode Driver IC. This column corresponds to the keys [-] [7] [8] or [9]. Why this appears to blank to led display during the fault I'm not sure.

Does this happen for all displayed digits or just ones with segA lit (top horizontal LED segment)

For example, does it happen with only [1] or [4] shown in the display.

cheers

Tony

[Tom Flatterhand]

(04-07-2022 06:25 AM)teenix Wrote:  Does this happen for all displayed digits or just ones with segA lit (top horizontal LED segment)

I have to thank! :-)

You have a correct guess. If one of the digits 1 or 4 are in the display, then the upper horizontal segment is not lit for these digits! Generalized: The upper segment lights only in the empty digits.

[teenix]

Sorry, I should have asked in previous post, do the keys in that row work ok? (- 7 8 9)

Have you checked inside for corrosion?
While it is open, you might try wiggling the circuit boards and reassemble. Might be a dodgy connection pin.

Electrically, it is hard to see how all segments light. Being a multiplexed display and seeing as how they are dimly lit suggests that they are somehow strobed on and off through the faulty row. An oscilloscope would be the only way to confirm what is happening.

You could disconnect the keyboard and see if there are any shorts or partial shorts in the keyboard matrix, especially when one of those keys is pressed.

With a bit of fiddling, with the keyboard disconnected, you could short the faulty row to a key column line. If it works ok, then the keyboard would appear to be the culprit. If not, it is more likely in the logic somewhere.

Current through the LEDs for each digit should only go through the active switch in the cathode driver IC. This is also connected to the keyboard. In the absence of anything visual, maybe this has gone faulty.

cheers

Tony

[Tom Flatterhand]

Hello Tony,

(04-07-2022 11:04 AM)teenix Wrote:  Sorry, I should have asked in previous post, do the keys in that row work ok? (- 7 8 9)

Have you checked inside for corrosion?
While it is open, you might try wiggling the circuit boards and reassemble. Might be a dodgy connection pin.

yes, the keys work properly. It is only the faint glow of the upper segments that bothers me.

It would be the first time I've opened a Woodstock calculator. I know that under the top rubbers are the screws. But my rubbers look fine so far. How safe is it if I try them out with a screwdriver that they won't get damaged in the process?

On the other hand, I would really like to see inside and try some of your tips.

See you

Thomas

[Maximilian Hohmann]

Hello!

(04-07-2022 01:04 PM)Tom Flatterhand Wrote:  How safe is it if I try them out with a screwdriver that they won't get damaged in the process?

Until 10 years ago (or so) the rubber feet could easily be pulled out without damage. But in my experience they now have aged to such a degree that usually under the hard top layer there is not much left but crumbles of synthetic rubber. Some turn into rather sticky stuff reminiscent of old chewing gum, it probably depends on the storage conditions.
Sooner or later they will fall out by themselves anyway, so you can as well remove them.
To replace them I cut "new" ones from a white eraser (similar to this one: https://www.amazon.de/Staedtler-Radierer...1671655517) using a sharp knife. With sufficient carving skills one could even replicate the orginial mushroom shape.

Regards
Max

[Tom Flatterhand]

Hello Max,

(04-07-2022 01:20 PM)Maximilian Hohmann Wrote:  But in my experience they now have aged to such a degree that usually under the hard top layer there is not much left but crumbles of synthetic rubber. Some turn into rather sticky stuff reminiscent of old chewing gum, it probably depends on the storage conditions.

i have attached 2 pictures. Can you tell from the pictures what condition they are in?

When I got the HP-25, the feet were more or less nasty black. I cleaned them with gasoline and now the computer sits really well on the table or even once on a sloping dashboard of a car, where it did not slip away even in curves.

A first attempt with a screwdriver I have broken off again, because I had nevertheless concerns that the rubber breaks with further levering in pieces. :-)

   
   

Thank you for the rubber link.

Greetings

Thomas

[Maximilian Hohmann]

Hello!

(04-07-2022 03:08 PM)Tom Flatterhand Wrote:  i have attached 2 pictures. Can you tell from the pictures what condition they are in?

From your pictures it looks as if they are a bit mushy or soft. I can see some dust sticking to them. So I don't think they will come out in one piece. I have not tried that yet, but maybe after an hour in the freezer the rubber hardens sufficiently to be pulled out undamaged.
Alternatively you could just drill a small hole in each them (2 or 2.5mm should be sufficient) to access the screws. A small Phillips screwdriver is required for that. That would leave most of the rubber feet in place.
The screw holes are not in the centre of the rubber feet but a little offset to the center of the calculator. I attach a photo below (this is the rubber foot on the right hand side, the left one is just mirrored).

Regards
Max

[Tom Flatterhand]

(04-07-2022 03:59 PM)Maximilian Hohmann Wrote:  I have not tried that yet, but maybe after an hour in the freezer the rubber hardens sufficiently to be pulled out undamaged.
Alternatively you could just drill a small hole in each them (2 or 2.5mm should be sufficient) to access the screws. A small Phillips screwdriver is required for that. That would leave most of the rubber feet in place.
The screw holes are not in the centre of the rubber feet but a little offset to the center of the calculator.

I'm impressed what expertise there is here about the beautiful old computers. The same goes for 'teenix'. I will play around with the computer a bit more. Let's see when the critical mass is reached and I finally decide to open it. The glowing segments are already annoying me a bit.

In your picture, it looks like the right side of the rubber mount is hollow. If I poke in there at an angle with a thinner screwdriver, I could reach the rubber quite far down and thus have good leverage....

I'll keep you posted...

Thank you both very much.

Thomas

[Tom Flatterhand]

Ok, I dared to do it. I got the rubbers out in one piece. Using a multimeter with continuity check, I identified the fourth contact from the top right of the keyboard as the one connected to the row of keys. This leads into the board where the LED unit is plugged in. I have marked the corresponding line in red in the picture. It seems to run a hair's breadth past the two contacts of the power supply and then to IC 1280-1382J 51147.

I plugged and unplugged all plug-in units several times, wiggled a bit everywhere, but nothing prevented that the upper segments did not turn on when pressing the buttons of the middle row.

   
   
   

[teenix]

Luckily, I don't think corrosion is an issue. :-)

I know you can get funny displays by pressing multiple keys at the same time, but it is hard to imagine the keyboard having a hard short as it would show up other operational problems. The same for the display module.

This seems to point to a chip problem.

As mentioned, because the display/keyboard operate in a multiplexed manner, an oscilloscope would help diagnose the problem. A shame I don't live around the corner, I would be happy to have a look.

Careful handling the board, the IC's are sensitive to static discharge coming from you.

cheers

Tony

[Tom Flatterhand]

Hello Tony,

there were traces of a leaking battery or accumulator on the battery contact on the right in the picture. I cleaned them with isopropyl alcohol.

(04-08-2022 12:01 AM)teenix Wrote:  As mentioned, because the display/keyboard operate in a multiplexed manner, an oscilloscope would help diagnose the problem.

I have a Fluke scopemeter. Could it be used to make the measurements? If you write me where to measure, I could try that.

Addendum: I read a bit in the beginning of the document 'Notes on HP's Classic Calculators'. There is a lot of knowledge in there! This is from you, isn't it?

Does this document also apply to the 'Woodstock' series?

Anyway, I think I have a rough understanding of how the LED display works and that it requires an interaction of different chips to display the characters correctly.

Nevertheless, the display is normally fine, if no key of the middle row is pressed. That means that the chips are all working properly! Only if this one line carries current, it comes to the weak shining of the upper segments in the display. This indicates that here the multiplexed signal for the LEDs contains an error.

The document says that there are 5 lines from the arithmetic chip to the anode driver chip. The signal is coded at this point and if the code is not correct here, it could cause such an error, right?

Possibly there is a short circuit or 'overvoltage' here that causes the code to be incorrect?

Regards

Thomas

[teenix]

(04-08-2022 05:38 AM)Tom Flatterhand Wrote:  I have a Fluke scopemeter. Could it be used to make the measurements? If you write me where to measure, I could try that.

Sounds like a plan.

I'd start by testing the output from the ROM0 chip. (5061-0430)
Pin number 1 is the A segment for the display.
I haven't had to work on a Woodstock yet so I assume with the calculator opened, you can connect the battery, or maybe an external power supply set to 2.5V.

The scope earth lead should go the Battery -ve, and the actual probe to pin # 1.

With just a 7 displayed, you should see a positive pulse of about 40uS. This should repeat once at roughly 3.8mS intervals. The rest of the time, the signal should not show much. A similar signal will appear for segments b and c (ROM0 pins 3 and 4). The other segments should not show anything except some noise. (pins 2, 6, 7, 8, and 9)

All the A segments in the entire display "see" the 40uS ON pulse, but only one digit should have a path to ground to let the current flow. This is the job of the Cathode Driver, providing this path for each digit in turn. For the same LEDs to light in other digits, there must be some path that allows current flow. Just a matter of finding it.

Of course, ROM 0 could be outputting erroneous data as well - hard to say.

With one of the keys pressed, you might notice some extra signals appearing.

If this causes ROM0 to drive the A segments for the other digits, you will see that same pulse every 320uS or so, except for digit 4 which as you mention is blank.

I don't think this is happening however, as these segments are dimmer.

In the faulty state, these are probably not be on all the time, but are probably strobed on and off at some point in time during the display refresh. Having a dual channel scope will help here because you can trigger the scope on the RCD pulse appearing at the Cathode Driver IC (1820-1382) pin 16. This is a small negative going pulse of 10 or so uS and resets the cathode driver to start the display scan again every 3.8mS or so. This way you can see what is happening over the full display refresh.

You could also test the STR input pin on the Cathode Driver (pin 10). This should pulse every 320uS or so and steps the Cathode Driver to the next digit. This might be worth monitoring when one of the keys is pressed. There should be no changes to the signal.

This will give you something to start with.

The Cathode Driver IC seems to have the same HP serial number as other Woodstocks so is probably compatible with other models if a swap over is required.

cheers

Tony

[Tom Flatterhand]

Before I do that, did you see that I added to the previous message in this thread? There I made an assumption. Can that be so?

[Tom Flatterhand]

I can't find the ROM0 chip. So, I can't find a chip labeled 5061-0430.

Another observation: in the picture of me shining a flashlight from behind on the circuit board where the two chips can be seen. It seems that the 4th contact from the left of the contact strip with the keypad has a direct connection with the 11th contact of the contact strip to the LED display. This is very strange to me since it is this contact of the keypad that connects to the middle row of keys? What is this directly to the display?

   

[teenix]

(04-08-2022 06:44 AM)Tom Flatterhand Wrote:  Before I do that, did you see that I added to the previous message in this thread? There I made an assumption. Can that be so?

Those Classics used a different approach to controlling the display. Externally, the Woodstock approach is simpler. !2 digits, one digit active at any time, and also only 1 LED segment ON at anytime. 40uS on x 8 segments = 320uS x 12 digits = 3.8mS per full refresh.

The 8 segments are output from the ROM0 chip and the ACT controls the Cathode Driver.

Have a look at the HP-29C repair manual starting from page 2-1. Same display deal for all the Woodstocks.

If you don't have it...

https://literature.hpcalc.org/community/hp29c-sm-en.pdf

cheers

Tony

[Tom Flatterhand]

In the repair book I could see in the wiring diagram that the wires are actually connected like i has written in this thread. :-) So there is definitely the possibility of influence. But probably I am on a wrong track.

I did not want to be smarter with my guesses in any case than the established experts here. Just to show that I am dealing with my problem myself. As a beginner in the HP calculator world this could not lead far of course. :-)

I am very happy about the good helps, but I can't always use them completely, because I have only one measuring line for my old scopemeter and also not enough crocodile clips. In addition, it did not start this morning at all, because it was for days without power supply connection and therefore probably the battery extremely emptied. I already thought it was broken. After meanwhile about 3 hours of charging, however, at least some power-on relay inside clicks again. Only the backlight is still off. But that was also in the past and therefore I'm in good spirits that it will fully work again today during the day.

The ROM0 is probably U1 from the location plan in figure 4-7 of the repair manual. According to the layout plan, the HP-34C is built a bit different than the HP-25. At least there seem to be more IC's on the bottom of the board than on the HP-25.

I will of course keep you (Tony) and the other readers informed...

Thomas

[teenix]

(04-08-2022 07:10 AM)Tom Flatterhand Wrote:  I can't find the ROM0 chip. So, I can't find a chip labeled 5061-0430.

Another observation: in the picture of me shining a flashlight from behind on the circuit board where the two chips can be seen. It seems that the 4th contact from the left of the contact strip with the keypad has a direct connection with the 11th contact of the contact strip to the LED display. This is very strange to me since it is this contact of the keypad that connects to the middle row of keys? What is this directly to the display?

Sorry, I was confused with the HP-27 problems. ROM0 in the 25 is part 1818-0168

You'll find many keyboard connections connected to the LED display. Both are multiplexed together to save pin counts on the ICs and keep the circuit boards simpler.

cheers

Tony

[teenix]

I'm not sure where the HP-34C came from, the Spice series have a different setup to the Woodstock.

I only mentioned the HP-29C manual so that you might see how the display and keyboard work which is explained in the manual and might help with a repair.

I attached a simple image of the keyboard display connections.

If you assume digit 4 is activated by the Cathode driver then any LEDs that are to be switched on for this digit will do so, but only one at a time. The LEDs are energized by the ROM 0 chip and the current flows into the Cathode Driver and then to ground. As all other digits are OFF, no LEDs in these will light during this time, unless there is a fault.

If no keys are pressed, the ACT will see all LOGIC 1's on the column inputs (1 - 5).

If, for example, the [7] key is held down continuously, when Digit 4 activates, the ROW 4 line on the keyboard is also connected to ground. Because ROW 4 and COL 3 are shorted together, the ACT will now see this LOGIC 0 in the ROW 3 line and it will generate a key code (98 in this case). The ACT will then execute code to process the 7 key.

When you release the key, the ACT will again only see LOGIC 1's on the column inputs all the time.

Now when digit 2 activates, the ROM 0 IC will output the segments (a b c and h) one at a time to display "7" and the decimal point.

Now, when you press any key on Row 4 (- 7 8 9) you see segment (a) dimly duplicated on all digits which is the fault. The trick is to find out why.

I hope this helps a bit more.

cheers

Tony

[Tom Flatterhand]

In the meantime, I was able to perform a few measurements. I measured the pins 1 to 9. Except for pin 5, I could detect a signal everywhere, which also repeated after about 3.8 ms. I took pictures of pin 1 while my daughter pressed button 9. I have attached 2 pictures of pin 1, one with the button pressed and one without.

   

   

Conclusion: I am not getting anywhere and will probably give up looking for a solution. Because except for the problem with the upper segments, the computer seems to work fine. And since the inside also looks very good, I will (probably) never try to unsolder any of the IC's there. Because this would end with my old equipment probably rather in a permanent damage.

Therefore many thanks to all helpers for your help! And of course especially to you Tony, you really made a lot of effort!

Now I need an old HP-25, one that is really damaged, so I can put in such a Panamatik chip. :-)

Background anecdote:

After 16 years I once again opened the case to my, at that time bought used, Scopemeter 123 and found out, that I have a second measuring line, as well as still small clamps. :-) And there was a cable for a COM port, which theoretically allows to establish a connection with the scopemeter via a purely optical connection. In the case is even the appropriate software for Windows 3.11!

I thought, great, then I can take pictures directly from the scopemeter display on the PC. That was also the idea. Because fortunately I even have extra interface cards installed in my Ryzen 3950! But unfortunately no floppy drive. OK, I thought, I'm sure I'll find something on the Internet and indeed I found it at Fluke:

https://www.fluke.com/en-us/support/soft...test-tools

I downloaded various versions and installed them under Windows 10. The software then tried various baud rates automatically, but unfortunately did not find a scopemeter. Finally I gave up and asked my daughter to take pictures of the screen with her smartphone. :-)